1. Field of the Invention
The present invention relates to a centrifugal compressor for an exhaust turbo charger, etc., a compressor housing of which has an inlet passage having a diameter larger than the diameter of an annular inlet area of the impeller of the compressor, and a plurality of slots are formed in the housing near the annular inlet area of the impeller so that gas introduced from the inlet passage can be introduced into the impeller through the slots at the outer periphery of the leading edge parts of the blades in addition to gas introduced into the impeller from the annular inlet area of the impeller or gas introduced from the annular inlet area can be bleed from the impeller through the slots to the inlet passage to be again sucked into the impeller from the annular inlet area, particularly a centrifugal compressor in which said plurality of slots are arranged circumferentially concentrically with the center of rotation of the impeller.
2. Description of the Related Art
A centrifugal compressor of an exhaust turbocharger has a stationary housing and an impeller supported for rotation in the housing, the impeller being rotated by a turbine rotor driven by exhaust gas of an engine. Air sucked in from the inlet passage of the housing is introduced into the impeller through the annular inlet area of the impeller, compressed therein by centrifugal force exerting on the gas sucked in the impeller, and discharged from the peripheral outlet area of the impeller to the outlet passage of the housing to be supplied to the engine therefrom.
It is demanded in the field of the centrifugal compressor of the exhaust turbocharger to broaden stable operation range of the compressor. In Document 1 (Japanese Laid-Open Patent Application No. 2004-27931) is disclosed a centrifugal compressor in which a plurality of slots are formed in the compressor housing near the annular inlet area of the impeller so that gas introduced from the inlet passage can be introduced into the impeller through the slots at the outer periphery of the leading edge parts of the blades in addition to gas introduced into the impeller from the annular inlet area of the impeller in an operation range of increased gas flow rate or gas introduced from the annular inlet area can be bleed from the impeller through the slots to the inlet passage to be again sucked into the impeller from the annular inlet area in an operation range of decreased gas flow rate, said plurality of slots being arranged circumferentially concentrically with the center of rotation of the impeller and formed to open to the spaces between the blades of the impeller at meridional distance from the leading edge of the blade in the range of 2˜21% of the meridional length along the contoured outer tip of the blade from the annular inlet area to the peripheral outlet area of the impeller, that is, from the leading edge to the trailing edge of the blade along the outer tip thereof, and said plurality of slots being formed between the inner surface of the inlet passage of the housing near the annular inlet area of the impeller and the outer periphery of an annular ring part, the inner periphery of which composes the outer periphery of the annular inlet area of the impeller, with the annular ring part supported by a plurality of struts extending from the inner surface of the inlet passage of the housing radially inwardly, thus the slots being partitioned by the struts.
However, there is a possibility that the noise produced by rotation of the impeller having a plurality of blades for compressing the gas sucked in the impeller, frequency of the noise being determined by the number of blades and rotation speed of the impeller, resonates with the vibration of gas in the slots, of which the natural frequency is determined by the length of the slot, and excessive noise is produced. Strength of the noise is influenced by the number of the struts partitioning the slots and circumferential location of the struts.